Heat storage apparatus

ABSTRACT

Heat storage apparatus comprising an insulated box containing a thermal energy storage body consisting essentially of a material selected from the group comprising carbon, magnesium oxide and mixtures thereof which may be in block form, sintered form and powdered form is disclosed. Means for heating the heat storage body and heat exchange means for transporting heat energy to an appropriate power system are described. Embodiments of the invention for operation at different temperatures are disclosed.

United States Patent Inventor [72] Charles Dewey Havill 3,388,205 6/1968 Haavik etal 13/23 22330 Holt Ave., Los Altos, Calif. 94022 3,2 1,203 11/1965 R gl n ,.1r 313/271 [21] AppLNo. 34,077 2,367,170 1/1945 Fahrenwa1d.... 219/540 [22] Filed May 4, 1970 1,069,374 8/1913 Bell et a1 219/54OX [45] Patented Nov; 30, 1971 2,911,513 11/1959 MacCracken 219/530X 2,670,425 2/1954 Stone 219/378 1,060,718 5/1913 Stan1ey..... 126/400 STRAGElAPl;\RATUS 3,179,736 /1965 Ramsey 13/25 11 Claims 4 Draw gs. Primary Examiner-Velodymyr Y. Mayewsky UQS. A"0rney Mein Moore and weissenberger 23/2092, 126/400, 165/180, 219/378, 219/540, 252/509 [51] Int. Cl 1105b 3/06 ABSTRACT: Heat storage apparatus comprising an insulated Field of Search 219/530, box containing a thermal energy storage body consisting es- 540, 365, 378, 430, 439, 462; /104, sentially ofa material selected from the group comprising car- 126/400; 313/271; 252/509; 23/2092 bon, magnesium oxide and mixtures thereof which may be in block form, sintered form and powdered form is disclosedv 1 References Clwd Means for heating the heat storage body and heat exchan e 8 UNITED STATES PATENTS means for transporting heat energy to an appropriate power 2,808,494 10/1957 Telwes 219/530 System are described- Embodimems of the invention for 3,381,113 4/1968 Jacques et a1 219/378 opefatkm atdifferent"empmmmm disclosedt l6 l2 l6 I \///F i/F x ::4 P/\ 491 //\l/ 1 1,1, 18 M, 15-

I/|/ l/ I 1 4 V I i 7 V 1 w I o 1 I /'j 5% a"? 44 7 N, 1,1 1%411 |I\\ HEAT STORAGE APPARATUS BACKGROUND or THE INVENTION This invention relates to energy storage systems and more particularly to apparatus for storing energy in the form of heat.

, In today's society with the environmental problems associated with air pollution, it is desirable to find a different source of energy for personal transportation systems, and other mobile power systems, which currently are polluting the atmosphere. Devices to accomplish this should store energy in a minimum weight system, be economically reliable relative to systems using combustion of hydrocarbons, and be extremely safe even under conditions where accidents occur an the storage device is ruptured.

The objects of the present invention are to provide an energy storage apparatus which is sufficiently light in weight, economical, and safe, to be competitive with combustion engines utilizing energy stored in hydrocarbon fuels. The present invention is similar to nature to many previous thermal energy storage systems, but utilizes materials and design characteristics intended to satisfy the foregoing requirements for a mobile system. Previous systems have been designed for high energy storage per unit volume, rather than per unit weight; for liquid phase systems utilizing the latent heat of fusion for energy storage, rather than solid body storage systems; and for special applications utilizing expensive materials, rather than inexpensive and readily available materials. Such prior design characteristics are adequate for fixed base systems which are primarily used to cancel incompatibility between power requirements and economic .power availability on a time scale, but do not satisfy the requirements for mobile systems involving personal transportation which are the object of this inventron.

SUMMARY OF THE INVENTION The apparatus of this invention consists of an insulated box containing the thermal energy storage body and including a heat exchanger means through which fluid is circulated to transport stored heat energy to the power system using it and means for heating the thermal energy storage body, the thermal energy storage body consisting essentially of a material selected from the group consisting of carbon or magnesium oxide or a mixture of magnesium oxide and carbon, either sintered or in the powder form. Various types of apparatus in accordance with this invention are possible. For example, a low temperature embodiment for peak storage temperatures below l,l50 C., and a high temperature embodiment for temperatures up to 2500 C. are described in detail herein.

DESCRIPTION OF THE DRAWINGS The objects and features of this invention will become more apparent from a reading of the following description of preferred embodiments thereof in conjunction with the attached drawing, wherein:

FIG. I is a plan view in cross section of an apparatus embodying this invention.

FIG. 2 is a cross-sectional side view in elevation taken along lines 22 of FIG. 1. 4

FIG. 3 is a plan view in cross section of an embodiment of this invention adapted for higher temperature operation than the embodiment shown in FIG. 1.

FIG. 4 is a cross-sectional side view in elevation taken along lines 4-4 ofFIG.3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, the apparatus of this invention includes a metallic container which is provided with a reflective inner surface. In the embodiment of FIG. 1, the container is shown as having a square configuration however, it will be understood that the container may be of any suitable configuration, including cylindrical and spherical configurations. It will also be understood that the reflective inner surface may be provided by any convenient means, including the lining of the interior of the container 10 with an appropriate layer of reflecting material, to minimize thermal radiation losses.

The thermal energy storage body 12 has the same configuration as the container 10 and is supported therein by means of two layers 14 and 15 of insulting material. The first insulating layer 14 comprises a material capable of withstanding high temperatures such as diatomatious earth, for example. The second insulating layer 15 need not be capable of withstanding as high a temperature as the first insulating layer due to the temperature drop across insulating layer 14. Thus the insulating material of layer 15 may be composed of well known low temperature insulating materials having high heat insulation efficiency.

As is best shown in FIG. 2, A plurality of heating elements 16 are embedded in the thermal energy storage body 12 and appropriate openings 17 are provided through the container 10 and insulating layers 14 and 15 to enable a source of energy to be connected to the heating elements [6. For example, the heating element 16 may comprise electrical heating elements to which a source of electrical energy is connected by means of openings 17 in order to generate heat within the thermal energy storage body 12.

Theheat stored in the thermal energy storage body 12 may be transported therefrom by circulating an appropriate fluid through a channel 18 in the body 12. For example, the channel 18 may comprise a tube, made of metal capable of withstanding high temperatures, embedded in the thermal energy storage body 12 with its ends 19 and 20 extending out of the container 10 through the insulating layers 14 and I5. It will be understood that any appropriate thermal energy utilization means may be connected to the ends 19 and 20 of the channel 18 to enable the energy stored in the thermal energy storage body 12 to be utilized as desired.

In accordance with the teaching of this invention, the thermal energy storage body 12 consists essentially of a material selected from the group consisting of carbon, magnesium oxide and mixtures of carbon and magnesium oxide. For example, the thermal energy storage body 12 may comprise a solid block of carbon or a solid block of magnesium oxide, in accordance with the teaching of this invention. However, in the preferred embodiment of this invention, the thermal energy storage body 12 consists essentially of a mixture of carbon and magnesium oxide in particulate form. If desired, such mixture may be sintered into a solid block. The principal advantages of using a proper mixture of carbon and magnesium oxide over materials previously used in heat storage systems are: (a) The heat storage capacity is much higher than all but a few very expensive and unstable materials. (b) The carbon and magnesium oxide will remain in a solid state in use so that the complex and expensive precautions required according to the teaching of the prior art to prevent liquid leakage and the attendant danger and thermal loss is avoided. (c) Carbon and magnesium oxide are sufficiently inert to that an accidental exposure to air at elevated temperatures will not result in a catastrophic explosion as might be the case with thermal storage materials of the prior art such as magnesium or lithium-hydride. (d) Carbon and magnesium oxide are readily available and inexpensive.

The embodiment of this invention shown in FIGS. 1 and 2 may be easily designed for operation at a temperature of about I, I 50 C. At such temperature, such embodiment will provide a heat storage capacity of approximately 250 watt-hours per pound of thermal storage material. According to the teaching of this invention, carbon is the preferred thermal storage material due to its high thermal conductivity and the fact that it may be heated by passing an electrical current directly therethrough. However, the use of pure carbon as the thermal storage material would present a safety hazard due to the rapid combustion which would tend to result if the carbon were accidentally exposed to air at high temperatures, particularly where the carbon is in powder form. Such safety hazard may be reduced by using carbon only in solid block form as the thermal storage material and further by coating it with magnesium oxide, for example.

Magnesium oxide does not present any safety problem since it is inert even in powder form. However, it has a relatively low thermal conductivity and is an electrical insulator. Thus if pur magnesium oxide is to be used as the thermal storage material, heat exchange problems will arise both in storing heat in the magnesium'oxide material and in removing heat from the magnesium oxide material. Such problems might be met by the provision of many minute heat exchange channels through the magnesium oxide body. However complicated and expensive techniques would be required in order to fabricate a suitable thermal energy storage body of pure magnesium oxide for most practical application.

For the above reasons, the thermal storage material of the preferred embodiment of my invention will comprise a mixture of magnesium oxide and carbon. The relative proportions of magnesium oxide and carbon will be primarily a function of the desired thermal conductivity and safety considerations. Generally the maximum safe percentage of carbon will be desired to increase conductivity and ease of extraction of heat for useful work. However, if the carbon consists of finely divided powder, then to 50 percent magnesium oxide should be added to inhibit an overly rapid reaction with atmospheric oxygen in the event of an accidental exposure to the thermal storage material at elevated temperatures. The percentage of magnesium oxide required to provide safety will depend on particulars of the specific storage device such as grain size and compactness of the mixture. Obviously, a sintered mixture of carbon and magnesium oxide would offer safety advantages. It will be understood that magnesium oxide has a higher heat capacity than carbon. Thus it will be seen that the addition of magnesium oxide to carbon will result in a body of increased heat capacity and if the particles of magnesium oxide are small enough and evenly dispersed throughout the carbon, the thermal conductivity of the body may approach that of pure carbon.

The fact that carbon may be heated by the passage of an electrical current directly therethrough is highly desirable and in the preferred embodiment of my invention the source of energy to be stored in the thermal storage material would be an electrical source and the heating elements 16 would comprise electrodes embedded therein. By connecting the electrical source across such electrodes 16 in an appropriate combination the entire volume of the thermal storage material may be quickly and efficiently heated by the passage of electrical current directly therethrough. Such electrodes 16 may be advantageously made of pyrolitic graphite rods having their grain axis so oriented as to provide maximum electrical conductivity and minimum heat conductivity along the axes thereof. Pyrolitic graphite bodies exhibiting maximum electrical conductivity and maximum heat conductivity at right angles to each other are well known in the art. Thus heat loss from the thermal storage material 12 through the openings 17 which provide access to the electrodes 16 may be minimized.

Referring to FIGS. 3 AND 4, an embodiment of this invention designed for operation at temperatures of about 2,500 C. with a heat storage capacity of approximately 750 watt-hours per pound is shown. As indicated by the use of the same reference numerals for identical parts, the embodiment of this invention shown in FIGS. 3 and 4 is basically the same as the embodiment shown in FIGS. 1 and 2, except for the addition of a further insulating layer 22 adjacent the thermal energy storage body 12. Such additional heat insulating layer 22 may comprise either foamed magnesium oxide insulation or pyrolitic graphite with the conducting axes thereof arranged for minumum heat conduction transversely through the layer. In addition, the channel 18 for the circulation of an appropriate fluid for transporting the heat stored in the thermal storage material 12 to an appropriate utilization device is provided by means of a slab 24 of carbon embedded in the thermal energy storage body 12 and having the channel 18 formed therein. Since the slab 24 of carbon is embedded in the thermal storage material 12, the danger of its accidental exposure to the atmosphere is greatly reduced. It will be understood that the channel 18 might be formed in the thermal energy storage body 12 where such material is a sintered block. It will also be understood that any material capable of withstanding the high temperatures of operation could be used to provide the tubular channel 18. In any Case, it is advantageous to use tubes of pyrolitic graphite designed to exhibit minimum heat conduction along their axes to provide the ends 19 and 20 of the channel 18 by which the ingress and egress of the heat transporting fluid through insulating layers 14, 15 and 22 is obtained.

It is believed that those skilled in the art will find many applications for devices embodying my invention and that such embodiments will be adapted in configuration and structure to suit the particular application for which they are designed by making obvious modifications in the embodiments described hereinabove. It will be understood that any suitable metal may be used for the container 10 and that any suitable insulating material may be used for the layers 14 and 15. Similarly, any suitable means of conducting heat into the thermal storage body 12 may be used as heating elements 16 although the use of carbon electrodes and an electrical source of energy for heating the thermal material 12 is preferred for the reasons set forth hereinabove.

I claim:

1. A heat storage apparatus comprising a thermal energy storage body, means for heating said thermal energy storage body, an insulating means comprising a container and insulating layers enclosing said thermal energy storage body for reducing heat loss from said thermal energy storage body, and means for extracting thermal energy from said thermal energy storage body, wherein said thermal energy storage body consists essentially of a material selected from the group consisting of carbon and mixtures of carbon and magnesium oxide.

2. A heat storage apparatus as claimed in claim 1 wherein said material is in solid block form.

3. A heat storage apparatus as claimed in claim 1 wherein said material is in particulate form.

4. A heat storage apparatus as claimed in claim 1 wherein said material is a mixture of carbon and magnesium oxidein sintered form.

5. In a heat storage apparatus comprising a thermal energy storage body, means for heating said thermal energy storage body, an insulating means for reducing heat loss from said thermal energy storage body and means for extracting thermal energy from said thermal energy storage body in a readily utilizable form, the improvement which consists in utilizing as said thermal energy storage body, a mass consisting essentially of a mixture of magnesium oxide and carbon in such proportions as to satisfy electrical resistance requirements for heating said body electrically, to satisfy thermal conduction requirements for heat extraction, and to satisfy safety requirements to avoid an explosive condition in the event of accidental breakage of the apparatus.

6. The apparatus of claim 5 wherein said insulating means comprises a layered thermally insulating container consisting of an inner layer of diatomaceous earth insulation, covered by an outer layer of lower temperature insulation, both contained within a thermally reflecting layer.

7. The apparatus of claim 5 wherein said means for-extracting the heat comprises high temperature metal tubing.

8. The apparatus of claim 6 wherein a further layer of insulation consisting of foamed magnesium oxide is provided internally of said layer of diatomaceous earth insulation.

9. The apparatus of claim 6 wherein a further layer of insulation consisting of a pyrolitic graphite exhibiting minimum heat conduction transversely thereof is provided internally of said layer of diatomaceous earth.

10. The apparatus of claim 5 wherein said means for extracting heat comprises a graphite slab with an internal channel for the flow of heat transfer fluid.

11. The apparatus of claim 5 wherein said means for extracting heat comprises a channel in saidthermal energy storage means, the ends of said channel extending through said insulating means and comprising pyrolitic graphite tubes designed for minimum heat conduction along the axis thereof. 5

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1. A heat storage apparatus comprising a thermal energy storage body, means for heating said thermal energy storage body, an insulating means comprising a container and insulating layers enclosing said thermal energy storage body for reducing heat loss from said thermal energy storage body, and means for extracting thermal energy from said thermal energy storage body, wherein said thermal energy storage body consists essentially of a material selected from the group consisting of carbon and mixtures of carbon and magnesium oxide.
 2. A heat storage apparatus as claimed in claim 1 wherein said material is in solid block form.
 3. A heat storage apparatus as claimed in claim 1 wherein said material is in particulate form.
 4. A heat storage apparatus as claimed in claim 1 wherein said material is a mixture of carbon and magnesium oxide in sintered form.
 5. In a heAt storage apparatus comprising a thermal energy storage body, means for heating said thermal energy storage body, an insulating means for reducing heat loss from said thermal energy storage body and means for extracting thermal energy from said thermal energy storage body in a readily utilizable form, the improvement which consists in utilizing as said thermal energy storage body, a mass consisting essentially of a mixture of magnesium oxide and carbon in such proportions as to satisfy electrical resistance requirements for heating said body electrically, to satisfy thermal conduction requirements for heat extraction, and to satisfy safety requirements to avoid an explosive condition in the event of accidental breakage of the apparatus.
 6. The apparatus of claim 5 wherein said insulating means comprises a layered thermally insulating container consisting of an inner layer of diatomaceous earth insulation, covered by an outer layer of lower temperature insulation, both contained within a thermally reflecting layer.
 7. The apparatus of claim 5 wherein said means for extracting the heat comprises high temperature metal tubing.
 8. The apparatus of claim 6 wherein a further layer of insulation consisting of foamed magnesium oxide is provided internally of said layer of diatomaceous earth insulation.
 9. The apparatus of claim 6 wherein a further layer of insulation consisting of a pyrolitic graphite exhibiting minimum heat conduction transversely thereof is provided internally of said layer of diatomaceous earth.
 10. The apparatus of claim 5 wherein said means for extracting heat comprises a graphite slab with an internal channel for the flow of heat transfer fluid.
 11. The apparatus of claim 5 wherein said means for extracting heat comprises a channel in said thermal energy storage means, the ends of said channel extending through said insulating means and comprising pyrolitic graphite tubes designed for minimum heat conduction along the axis thereof. 